This invention relates to castable refractory mixtures. More particularly, it relates to a castable refractory mixture having enhanced corrosion resistance to molten aluminum.
In the processing of metal such as aluminum and aluminum alloys for casting or purification, the metal is generally melted in a furnace and conveyed in troughs lined with suitable refractory material to inhibit attack by the molten aluminum. Calcium aluminate is known as a good refractory material from the standpoint of corrosion resistance to molten aluminum. However, calcium aluminate has a high thermal conductivity and a high thermal expansion coefficient.
It is known to modify calcium aluminate with other materials which counter the deleterious effects noted above. For example, Kadisch et al. U.S. Pat. No. 2,874,071 teaches a mixture of calcium aluminate with fused silica. The patentees further state that the material may be made much more refractory by an additional treatment involving dipping, spraying, or brushing the surface thereof with a plastic composition containing super-refractory powders or mixtures of such powders. The patentees state that such super-refractory powders are made by specially milling stabilized fused zirconium or alumina. They further state that refractory borides, nitrides, or carbides and mixtures thereof of silicon, molybdenum and chromium can also be applied as a coating.
While the use of calcium aluminate-silica mixtures is preferred to pure calcium aluminate because of the lower thermal conductivity of silica and the lower shrinkage of the mixture, the success of the mixture apparently depends on the bond between the calcium aluminate and silica which, if broken, permits metal attack of the silica. It is known that the use of boron oxide (B.sub.2 O.sub.3) together with silica and calcium oxide and aluminum oxide provides a refractory material which prevents metal attack of siliceous aggregates. However, the prior art incorporated the boron oxide into the mixture by providing a fused or glassy matrix of calcium oxide, boron oxide, and aluminum oxide For example, McDonald et al U.S. Pat. No. 2,997,402 teaches a refractory which comprises a homogeneous vitreous product and a refractory aggregate wherein the homogenous vitreous product is a fused mixture of calcium oxide, boron oxide, and aluminum oxide and the refractory aggregate is principally a mixture of aluminum oxide and silicon dioxide is comminuted form. McDonald et al also suggest that up to 15% by weight of a additional metal oxide component can also be incorporated in the glass such as the oxides of magnesium, barium, beryllium, zirconium, zinc, vanadium, silicon, chromium, and molybdenum.
Rubin et al U.S. Pat. No. 3,471,306 mixes bondforming components containing Al.sub.2 O.sub.3, B.sub.2 O.sub.3, and CaO with a pre-calcined granular grog containing SiO.sub.2 in excess of Al.sub.2 O.sub.3 to form a calcium boroaluminate bond in situ.
While the use of such materials can result in satisfactory mixtures, fritted or glassy matrices comprising calcium oxide, boron oxide and aluminum oxide are not easily formed. For example, in the formation of the homogeneous vitreous product of McDonald et al. care must be taken to minimize the water solubility of the frit wherein the boron oxide and the calcium oxide might otherwise be leached out. On the other hand, the formation of a calcium-boroaluminate bond in situ as in Rubin et al. is not easily achieved because of added complications in firing due to the presence of the aggregate (e.g. one cannot melt the bond-forming components as in McDonald et al to ensure formation of a homogeneous stable mixture).
It is therefore an object of this invention to provide a refractory mixture wherein a boron oxide-containing material is used in the form of a zinc borosilicate glass frit having a low content (less than 0.5% by weight) of impurities such as alkali and alkaline earth additives which could otherwise be leached out of the frit in aqueous systems commonly used to form refractories.